Method for controlling a radar antenna

ABSTRACT

A method is described for detecting an object with target radar. After the first detection of an object, the antenna is tilted. The original direction towards the object is restored by electronically steering the emitted beam. The beamsteering is obtained by changing the frequency of the emitted signal. Thus, the object may be detected several times on different frequencies. The method is particularly applicable in search and track radars.

TECHNICAL FIELD

The present invention relates to the field of radars for search andtracking.

TECHNICAL BACKGROUND

Currently, in radars searching and tracking the path of projectiles,successive rounds from the same gun or other trajectories will bedetected and tracked with the radar using the same frequencies eachtime. Thus both the detection probability and the tracking errors willbe correlated, since these depend on the frequency characteristics ofthe radar cross-section (RCS). RCS is the measure of a target's abilityto reflect radar signals in the direction of the radar receiver.

Solutions are known in which the antenna orientation/position of theradar are changed. However, changing the antenna orientation/radarposition will cause a period of time where the radar is notsearching/tracking.

Another solution is to use two or more radars. Using several radars toget better search and tracking is expensive.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method in which aradar can obtain a better ability to detect and follow an object withouthaving to relocate the radar or use more radar units.

This is obtained in a method as defined in the appended claims.

Briefly, the method consists of mechanically tilting the antenna andcompensating for the tilting with beamsteering, said beamsteering beingperformed by changing the frequency of the emitted signal. Forsuccessive rounds from the same gun or other trajectories that arerepeated, each round will be tracked using different frequencies. As aconsequence, the detection probability will increase and the trackingerror will be reduced, without the need for a break in operation time orthe use more than one radar.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail in reference to theappended drawings, in which

FIG. 1 shows an antenna that is being tilted in elevation,

FIG. 2 is a schematic diagram of the hardware of a radar system,

FIG. 3 is a sequence diagram showing the commands issued for changingthe mechanical orientation of the antenna,

FIG. 4 is a diagram showing the commands for adjusting the searchhorizon in view of the new antenna orientation,

FIG. 5 is a diagram showing the sequence for predicting the next trackpoint of a moving object,

FIG. 6 is an example of how a tracking system can be implemented in aDPU.

DETAILED DESCRIPTION OF THE INVENTION

We want to steer the beam at the same place for search but with adifferent frequency to get a different RCS. This can be done bymechanically tilting the antenna and compensate the tilt by electricallysteering, the beamsteering being performed by changing the frequency ofthe emitted signal. The compensation must be done while the antenna ismoving. To be able to do this, we must measure the antenna orientationat a high rate. It is important that the compensation is done while theantenna is moving so we can search and track continuously.

FIG. 1 shows the antenna at two different tilt angles, but we want tosteer the beam at the same target. To do this, we can change thefrequency so the electrical steering of the beam compensates for themechanical tilt difference. In the figure the electrically steering mustbe ψ and ψ′.

When the radar is tracking a target it shall predict the trajectory ofthe target to be able to steer the beam so it follows the target. Inaddition to this prediction, we must also take the antenna orientationinto account and predict the antenna orientation the next time wemeasure the target position.

A schematic diagram of the hardware is given in FIG. 2. The navigationsystem unit is located on the antenna and measures the antennaorientation. The turntable unit is responsible forphysically/mechanically changing the antenna orientation. The SignalProcessing Unit (SPU) and Data Processing Unit (DPU) can be located onthe same or different hardware. These units can be constituted by PCs,workstations or real-time systems. The division of work tasks betweenSPU and DPU is a matter of convenience; the SPU is often a fastprocessing front end unit realized in discrete electronics, while theDPU is a PC that handles the signal at a lower speed. As the processingrate of PCs increases, they will be able to handle faster signals andcan then take on some of the tasks from the SPU.

Sequence Diagrams

Steering the antenna orientation

We want the antenna to mechanically move continuously or with pausesbetween the movements. This is done by commanding the turntable unit tochange the antenna orientation, as shown in FIG. 3.

Compensate for Search

The data processing unit tells the signal processing unit where thesearch beams shall be. Because the antenna is moving, these search beamsmust be adjusted in elevation. Based on the antenna orientation, the DPUeither gives the SPU new search beams or a message with elevationoffsets for different elevation angles. This is shown in FIG. 4.

Compensate for Tracking

When the radar is tracking a target, we must steer the beam where it ismost likely to see the target at the next measurement. When the antennais moving, we must also take the antenna movement into consideration. Todo this we can find the position of the target when we receive a trackmessage from the SPU, and then predict where the target is at the nextmeasurement (predicted track point), estimate the antenna orientation atthe next measurement time, and finally, adjust the predicted track pointbased on the information on the antenna orientation. This is shown inFIG. 5. FIG. 6 shows one possible solution in the DPU.

While the invention has been described with an antenna that is tilted inthe elevation direction, the same principle can be used for adjustingthe beam in any other direction.

1. A method in a target tracking radar installation, for detectingprojectiles following a trajectory, said projectiles being fired inrounds from a gun, said method comprising: sending a radio signal at afirst frequency in a beam towards a projectile from a multi elementantenna, receiving and registering a signal from the first frequencyreflected by said projectile, mechanically tilting the antenna;compensating for the mechanical tilt by changing to a second frequencyof the radio signal; and sending, receiving and registering subsequent,reflected radio signals sent back from projectiles following saidtrajectory in subsequent rounds.
 2. The method as claimed in claim 1,further includes further comprising the steps of: determining the tiltedposition of the antenna and calculating how much the frequency must bechanged in order to restore the correct position of the beam.
 3. Themethod as claimed in claim 2, wherein the radar includes a navigationsystem for detecting the current position, speed and acceleration of theantenna and for calculating a new anticipated position of a projectileand steering the beam towards t the new anticipated position.